What caused the hot mudflows during the mount saint Helens eruption

The glaciers on the mountain melted rapidly due to the heat of the eruption. The water from the glaciers quickly then rushed down the side of the mountain, picking up all sorts of debris, amking hte water mudy

A mudlow caused by a volcanic eruption is called a lahar

Mount St Helen's is a condensed volcano and when the explosion happened hot Lahaar spewed out and melted the ice/ snow in the area, hence the flood.